Reference is made FIG. 1 showing a light emitting diode (LED) circuit 10. The circuit 10 includes a load circuit 12 formed by a string of series connected light emitting diodes (LEDs) 14. The string is connected between a node 16 and node 18. Node 16 may correspond to a supply bus in an implementation of the circuit 10 including plural strings of LEDs. A voltage regulator 20 is coupled to receive an unregulated supply voltage Vsup and operates to generate a fixed regulated bus voltage Vbus that is applied to the supply bus at node 16. A current sink driver circuit 22 is connected to node 18 and operates to regulate a constant load current IL through the load circuit 12 formed by the string of LEDs.
Those skilled in the art understand that there is a minimum voltage Vmin for the bus voltage Vbus that is necessary in order to be able to regulate the constant current IL in the string. That minimum voltage Vmin is set by the following equation:Vmin=VL+Vmin-drop where: VL is the voltage drop across the load formed by the string of LEDs from node 16 to node 18, and Vmin-drop is minimum required “drop-out” voltage across the channel of the MOSFET device within the current sink driver circuit 22 needed to maintain a regulated output for the load current IL.
As the circuit 10 ages, the values for VL and Vmin-drop increase. This results in a corresponding increase in the value for Vmin. Accordingly, circuit designers will choose a value for the fixed regulated bus voltage Vbus that is higher than the anticipated range of Vmin over the life of the circuit 10. A side effect of making this choice for the regulated bus voltage Vbus value is an excess power dissipation (equal to (Vbus−Vmin)*IL). In higher power load situations, the loss associated with this excess power dissipation can be excessive.
A need accordingly exists in the art for circuitry to dynamically maintain the regulated bus voltage generated by the voltage regulator at a value which substantially equals (or slightly exceeds) the instantaneous minimum voltage Vmin necessary to achieve a regulated constant current IL in the string.